The present invention relates to a stencil printer or similar printer and more particularly to a printer capable of printing a multicolor image by conveying a paper or similar recording medium via consecutive print drums only once.
It is a common practice with a stencil printer to arrange a plurality of print drums each storing ink of particular color in the direction of paper conveyance. While a paper is conveyed from the upstream side toward the downstream side of the printer once, images of different colors are sequentially transferred from the print drums to the paper one above the other. As a result, a multicolor image is printed on the paper by a single pass of the paper. Such a single pass system is far more efficient than a system requiring a print drum to be replaced color by color and requiring a paper to be repeatedly fed. However, the single pass system has an offset ghost problem ascribable to a short distance between consecutive print positions.
Specifically, in the single pass system, a paper carrying an image transferred from an upstream print drum, e.g., a first-color print drum is brought to a downstream print drum, e.g., a second-color print drum without ink forming the image being dried. As a result, the ink is transferred from the paper to a master wrapped around the downstream drum and therefore from the master to the next paper.
The transfer of wet ink from the paper to the master wrapped around the downstream print drum is not critical. However, the ink transferred from the paper to the above master is again transferred to the next paper carrying an image of a first color transferred from the upstream print drum (so-called retransfer). The retransfer does not degrade image quality if the ink can be retransferred to the next paper in accurate register with the image of the first color printed on the paper. The retransferred ink, however, forms an offset ghost and critically degrades image quality if deviated from the image carried on the next paper. For example, for a given deviation, the offset ghost renders thick lines blurred and thin lines doubled.
While the retransfer cannot be obviated in the single pass, multicolor printer, the offset ghost ascribable to the deviation of a retransfer position can be highly accurately controlled if the upstream and downstream print drums rotate in accurate synchronism with each other and if papers are conveyed with high accuracy.
To control the offset ghost, it has been customary to drive the upstream and downstream print drums by interlocking them to each other. Japanese Patent Laid-Open Publication No. 4-329175, for example, teaches an interlocked drive system in which the shafts of the print drums are interconnected by a plurality of gears. Japanese Patent Laid-Open Publication No. 7-17121 proposes another interlocked drive system using timing pulleys and a timing belt.
However, the conventional interlocked drive system, whether it be the gear scheme or the timing belt scheme, has a problem that the gears, timing belt and other rotatable members interlocking the print drums are more or less eccentric for machining reasons and therefore vary their speeds during one rotation. As for the gear scheme, high rigidity available with a gear train can reduce the deviation of the offset ghost if high precision gears are used. However, a plurality of high precision gears increase the production cost of the printer. The printer with the timing belt scheme is low cost because use can be made of inexpensive timing pulleys that can be produced by, e.g., injection molding on a quantity basis. However, the timing belt and timing pulleys involve eccentricity and aggravate the deviation of the offset ghost.
Technologies relating to the present invention are also disclosed in, e.g., Japanese Patent Laid-Open Publication No. 11-129600.
It is therefore an object of the present invention to provide a printer capable of reducing the deviation of an offset ghost without increasing the cost.
In accordance with the present invention, a printer includes a plurality of print drums spaced from each other in a direction in which a recording medium is conveyed. A plurality of rotatable members interlock the print drums with respect to drive. The print drums and rotatable members are so arranged as to prevent an upstream and a downstream print drum rotating synchronously to each other from being brought out of synchronism when the recording medium arrives at the downstream print drum.
Also, in accordance with the present invention, a printer includes a plurality of print drums spaced in a direction in which a recording medium is conveyed. A plurality of toothed drum drive pulleys each are mounted on a particular print drum. A timing belt is passed over the drum drive pulleys to thereby inter lock the print drums with respect to drive. A phase adjusting device includes adjustment pulleys meshing with the timing belt, and displaces the adjustment pulleys for adjusting a phase between the print drums. The adjustment pulleys each have a number of teeth which is 1/integer of the number of teeth of each drum drive pulley.
Further, in accordance with the present invention, a printer includes a plurality of print drums spaced in a direction in which a recording medium is conveyed. A plurality of toothed drum drive pulleys each are mounted on a particular print drum. A timing belt is passed over the drum drive pulleys to thereby interlock the print drums with respect to drive. A phase adjusting device includes adjustment pulleys meshing with the timing belt, and displaces the adjustment pulleys for adjusting a phase between the print drums. Steer pulleys are fixed in place between the drum drive pulleys and the adjustment pulleys and contact the rear of the timing belt for steering it. The steer pulleys each have a pitch circle diameter which is 1/integer of the pitch circle diameter of each drive pulley.
Moreover, in accordance with the present invention, a printer includes a plurality of print drums spaced in a direction in which a recording medium is conveyed. A plurality of toothed drum drive pulleys each are mounted on a particular print drum. A timing belt is passed over the drum drive pulleys to thereby interlock the print drums with respect to drive. A phase adjusting device includes adjustment pulleys contacting the rear of the timing belt between the print drums, and displaces the adjustment pulleys for adjusting a phase between the print drums. The adjustment pulleys each have a pitch circle diameter which is 1/integer of the pitch circle diameter of each drum drive pulley.
In addition, in accordance with the present invention, a printer includes a plurality of print drums spaced in a direction in which a recording medium is conveyed. A plurality of toothed drum drive pulleys each are mounted on a particular print drum. A timing belt is passed over the drum drive pulleys to thereby interlock the print drums with respect to drive. A phase adjusting device includes adjustment pulleys contacting the rear of the timing belt between the print drums, and displaces the adjustment pulleys for adjusting a phase between the print drums. Steer pulleys are fixed in place between the drum drive pulleys and the adjustment pulleys and meshing with the timing belt for steering it. The steer pulleys each have a number of teeth which is 1/integer of the number of teeth of each drive pulley.